iOS磁力传感器.png

有三种获取磁力计数据的方法，如上图所示

1. Core Motion框架 CMMotionManagers的CMMagnetometer类

2. Core Motion框架 CMDeviceMotion的CMCalibratedMagneticField属性

3. Core Location框架 CLLocationManager的CLHeading

1.CMMagnetometer

来自磁力计的原始读数

 *  magneticField
 *  
 *  Discussion:
 *    Returns the magnetic field measured by the magnetometer. Note
 *        that this is the total magnetic field observed by the device which
 *        is equal to the Earth's geomagnetic field plus bias introduced
 *        from the device itself and its surroundings.
 */
@property(readonly, nonatomic) CMMagneticField magneticField; 

2.CMDeviceMotion（CMCalibratedMagneticField *）

磁场磁场 计读数针对器件偏差（板载磁场）进行了校正，没有器件偏差。

/*
 *  magneticField
 *  
 *  Discussion:
 *          Returns the magnetic field vector with respect to the device for devices with a magnetometer.
 *          Note that this is the total magnetic field in the device's vicinity without device
 *          bias (Earth's magnetic field plus surrounding fields, without device bias),
 *          unlike CMMagnetometerData magneticField.
 */
@property(readonly, nonatomic) CMCalibratedMagneticField magneticField NS_AVAILABLE(NA,5_0);

CMCalibratedMagneticField包括两个字段:field(XYZ轴上的磁场强度)和accuracy(磁场长度的精度)。

3.CLHeading [x | y | z]

针对器件偏置校正磁性计读数并过滤以消除局部外部磁场（通过器件移动检测 - 如果器件随器件移动，忽略它;否则测量）

/*
 *  CLHeading
 *  
 *  Discussion:
 *    Represents a vector pointing to magnetic North constructed from 
 *    axis component values x, y, and z. An accuracy of the heading 
 *    calculation is also provided along with timestamp information.
 *  
 *  x|y|z
 *  Discussion:
 *    Returns a raw value for the geomagnetism measured in the [x|y|z]-axis.

CLHeading是“ 偏离设备跟踪的磁场线”，而magnetometerData.magneticField是设备观察到的总磁场，等于地球的地磁场加上设备本身及其周围的偏置“ 。

因此，CLHeading会提供经过滤的值，其中“参照系”是现有的地球磁场。而磁力计数据提供未经过滤的值，参考帧是设备。

例子

#import <CoreMotion/CoreMotion.h>
@property (nonatomic, strong) CMMotionManager * motionManager;

- (void)viewDidLoad {
    [super viewDidLoad];
    _motionManager = [[CMMotionManager alloc] init];
    
    // 判断设备是否支持磁强计
    if (_motionManager.magnetometerAvailable){
        [_motionManager startMagnetometerUpdates];
        NSLog(@"该设备支持磁强计");
    }else{
        NSLog(@"该设备不支持磁强计");
    }

    // 磁强计 开启
    [_motionManager startMagnetometerUpdates];
    [self useMagnetometerPush];
}

// 使用第二种CMDeviceMotion的CMCalibratedMagneticField属性
- (void)useMagnetometerPush{
    // 设定获取间隔时间
    _motionManager.magnetometerUpdateInterval = 0.5;//0.1f;
    // 添加线程
    NSOperationQueue * queue = [[NSOperationQueue alloc]init];
        
    [_motionManager startDeviceMotionUpdatesUsingReferenceFrame: CMAttitudeReferenceFrameXArbitraryCorrectedZVertical toQueue:queue withHandler:^(CMDeviceMotion * _Nullable motion, NSError * _Nullable error) {
        NSLog(@"磁强计X = %.04f 磁强计Y = %.04f 磁强计Z = %.04f   精度:%d",motion.magneticField.field.x,motion.magneticField.field.y,motion.magneticField.field.z, motion.magneticField.accuracy);
    }];
}

// 通过第一种CMMotionManagers的CMMagnetometer类获取原始数据
- (void)useMagnetometerPushOriginal{
    _motionManager.magnetometerUpdateInterval = 0.5;//0.1f;
    NSOperationQueue * queue = [[NSOperationQueue alloc]init];
        
    [_motionManager startMagnetometerUpdatesToQueue:queue withHandler:^(CMMagnetometerData * _Nullable magnetometerData, NSError * _Nullable error) {
        NSLog(@"磁强计X = %.04f 磁强计Y = %.04f 磁强计Z = %.04f",magnetometerData.magneticField.x,magnetometerData.magneticField.y,magnetometerData.magneticField.z);
    }];
}

最后

如果你想知道哪里是磁性或真正北方的位置，推荐使用CLHeading，可以做指南针的一类应用。如果想要创建一个响应紧邻设备磁场的应用，或者想要执行某种特定的传感器融合，可以尝试创建一个AHRS，然后使用CMMagneticField即第二种方式。